i suppose you could put a turbo in but could you imagine what would happen the first time you hit the water?, still a 500 is no slouch but there really is no comparison to cc's. Muffler would help but that just pisses off the guys who want to hear the sound of nature.
My uncle put an hmf and a dalton clutch kit and the bike is pretty impressive. I just put a muzzy exhaust on my 650 and I am happy with that as well. The 500 is an impressive bike I think a couple bolt ons will wake it up pretty good.
Ok an hmf swamp exhaust slip on will wake up throttle response and give u up to 3 HP. A custom air filter ( twin air) can give u free HP by getting rid of the 90 bend in the factory filter. A clutch kit will help transfer the most power to the tires